In B-mode ultrasound imaging, two-dimensional images of tissue are created in which the brightness of a pixel is based on the intensity of the echo return. During conventional two-dimensional imaging, gain adjustments provide overall image changes. The gain is typically adjusted after beamforming and before signal processing, i.e., prior to envelope detection. Gain adjustment in the axial direction, known as "time gain compensation" (TGC), is carried out by increasing or decreasing gain as a function of depth. In addition, "lateral gain compensation" (LGC) can be used to adjust the gain setting as a function of lateral position.
The TGC block at the output of the beamformer is basically a depth-dependent gain control designed to compensate the received signal to correct for the attenuation caused by tissues at increasing depths. It is often set based on a nominal tissue attenuation factor (e.g., 0.5 dB/cm-MHz) and beam diffraction losses as a function of depth. The objective is to produce uniform tissue image brightness from the near field to the far field. In practice, the tissue attenuation properties may deviate from the assumed constant factor (or an application-dependent internal TGC curve), and may vary significantly with depth, especially if macroscopic structures and reflectors are present. Further, if the far-field regions are very noisy, it is desirable to suppress their pixel intensities for best overall image presentation. For these reasons, manual TGC adjustment is usually provided via a column of "slide pots" (potentiometers) or rotary knobs on the front panel, for different depth zones. The externally adjusted TGC for different depth zones is usually graphically displayed as a TGC curve in the monitor display. The TGC graphic is generated, for example, by a graphic processor as an overlay to the image display.
For cardiac sector imaging, the cardiac tissues/borders that run parallel to the ultrasound beam often do not produce strong echoes. Therefore, in addition to TGC, LGC adjustment has also proven useful for boosting cardiac borders within selected image sectors, while leaving the chambers dark. LGC allows the user to control gain in the lateral plane by adjusting the gain setting as a function of lateral position. For example, gain is controlled in small user-selected sectors across the image. LGC can be implemented at the same point as TGC in the B-mode processing path. A graph of the LGC curve similar to the TGC curve is also often displayed on the video monitor.
While state-of-the-art scanners provide the user with a host of selectable imaging parameters, including transmit frequency, acoustic output, external TGC and LGC controls, frame averaging level, dynamic range, edge-enhancing filters and video gray mapping, all of which can significantly affect the sensitivity, uniformity and feature enhancements of an image, the sonographer usually does not have the time (or training) to fully optimize all the available controls. To improve the ease-of-use and efficiency of the ultrasound examination, there is a need to automate some of the basic imaging parameter selection based on actual image data.